1. Field of the Invention
The present invention relates to automatic transmissions, in particular for automobile vehicles, comprising planetary gearsets controlled by friction members such as clutches and brakes, and usually coupled to a start-up coupling device subject to slip, such as a torque converter generally provided with a lock-up clutch, or sometimes a simple clutch.
2. Description of the Related Art
The recent development of these transmissions shows a marked tendency to increase the number of speeds, quite commonly six. The patent EP 0434525 B1 has described such a transmission comprising, between an input shaft and an output shaft housed in a casing, a so-called double planetary gearset with four members identified from first to fourth in order of speed, concentric with the output shaft, and at least one power path between the input shaft and the double planetary gearset, and five control devices, that is to say three clutches and two brakes of which the selective operation in pairs determines various transmission ratios between the input shaft and the output shaft, a first power path having a first fixed speed ratio and including a first clutch and a second clutch, and a second power path having a second fixed speed ratio, higher than the first fixed speed ratio and in the same direction, and including a third clutch. The first member of the double planetary gearset is connected to the first power path by the first control clutch. The second member is fixed to the output shaft. The third member is connected to the second power path by the third control clutch and held against rotation by the first brake. The fourth member is connected to the first power path by the second control clutch and held against rotation by the second brake. Selective operation in pairs of the five control devices provides six forward gears, i.e. a first, by the first control clutch and the first control brake, a second, by the first control clutch and the second control brake, a third, by the first and second control clutches, a fourth by the first and third control clutches, a fifth, by the second and third control clutches, a sixth by the third control clutch and the second control brake, a braked neutral position, by the first and second control brakes, and a reverse drive, by the second control clutch and the first control brake.
This transmission has been widely adopted by automobile vehicle manufacturers.
It has been proposed to modify the six-speed transmission of EP 0434525 in order to produce a transmission with seven or more speeds (see Prof.-Dr. Peter Tenberge, in Der E-Automat, Ein Automatikgetriebe mit Esprit, Dec. 2001). This transmission with seven or more speeds has serious drawbacks, in particular as regards the progression of steps of the ratios.